EP0058812A2 - Porous ceramic filter and process for producing the same - Google Patents

Abstract

The present invention resides in an improved ceramic foam filter particulary useful for filtering molten metal. The composition of the ceramic foam material is as follows: 50 to 70% Al2O3; 2 to 10% micron sized reactive alumina; 1 to 5% montmorillonite; 1 to 10% ceramic fibers; and from 5 to 25% of a ceramic binder or air setting agent.

Description

_U eblicherweise contains molten aluminum solid inclusions that are detrimental in a finished part quality and therefore undesirable. These solid inclusions usually come from three sources. Part of the inclusions are particles of aluminum oxide. The aluminum oxide comes from the oxide film floating on the molten aluminum. Other inclusions are fragments of the furnace lining, the sprue and other parts that are attacked by the molten aluminum and carried away with the flowing metal. Other particles are inclusions of insoluble impurities, such as intermetals, borides, carbides, or precipitates from other aluminum compounds, such as chlorides. If these inclusions remain in the finished casting after the metal has solidified, they cause reduced ductility or poor machining characteristics. Therefore, it is desirable to remove these solid inclusions from the stream of molten aluminum before it has been cast into a solid article which is used as such or processed by deformation such as rolling, forging, extrusion, etc. Filtering procedures to remove solids from liquids are carried out by passing the solids-laden liquid through a porous filter medium that retains the solids.

Filtering molten metal in general and molten aluminum in particular creates new problems because the liquid is so aggressive that it is difficult to find a stable filter medium. Generally two methods are used to remove inclusions from molten aluminum and its alloys before casting. The best known filter medium is the open glass fiber fabric, which is in the area of the transport or pouring channel or in the area of the liquid metal above the solidifying metal block in the mold pouring is arranged. However, such fabrics are only able to remove relatively large inclusions from the metal and are easily vulnerable, since the glass fibers soften strongly at the temperatures of molten aluminum. It is also known to filter molten aluminum through a bed of loose aluminum oxide bodies, for example tabular aluminum oxide. This method has the usual disadvantages of bed filters in that too little solid is retained. Such filters tend to work inefficiently due to channel formation, the pore size is difficult to set and check and an initially correctly set pore size can change slightly during use of the filter. In addition, if the filter is not in use, the metal contained in it must be kept liquid. An improved method for removing inclusions is known from US Pat. No. 3,947,363 and consists in filtering molten aluminum by means of a porous ceramic filter consisting of a plurality of interconnected cavities surrounded by a ceramic network. The composition of such filter materials are aluminum oxide-chromium oxide mixtures of preferably 45 to 55% A1 ₂ 0 ₃ , 10 to 17% Cr ₂ 0 _{3 '} 0.5 to 2% bentonite and 12 to 17% of a ceramic binder, such as aluminum orthophosphate. The use of the porous ceramic filter according to US-PS

3 947 363 led on the one hand to a greatly improved filtration process, on the other hand the use of relatively large amounts of chromium oxide in the filter formulation was disadvantageous. The high price of the chromium oxide, especially compared to the aluminum oxide, the limited availability of the chromium oxide and the delivery difficulties resulting therefrom are particularly disadvantageous. Another problem is the realization that Cr can be carcinogenic. Although such filters do not contain any measurable amounts of Cr ⁺⁶ , the use of chromium oxides must be restricted.

It has also been found that the production of large-area filters in the preferred thickness of round 5.5 cm strength problems can result. These ^- strength problems could be solved by thicker filters, but with the same filtering effect such filters would be much more expensive. It was therefore desirable on the one hand to increase the strength inherent in the ceramic composition and at the same time to eliminate the chromium oxide therefrom. Another important aspect was also to propose a filter which is so strong that it can be used to filter melts of different metals, which can be produced at an acceptable cost and which has excellent filter properties.

The object of the present invention was therefore to propose a filter which, in the absence of the disadvantages described, has the desired properties.

According to the invention this is achieved with a filter according to claim 1.

The porous ceramic filter according to the invention is particularly suitable for the filtration of molten metals and in particular for the filtration of molten aluminum. With the high temperature resistance that the filter has, it is particularly suitable for the difficult conditions that occur during metal filtration. It has further been shown that the composition of the ceramic filter does not result in any contamination of the metal by filter components. The ceramic filter according to the present invention is an inexpensive material for single use and subsequent replacement. The molten metal can be filtered with the ceramic filter in amounts of around 0.127 m ³ / m2 filter area to 12.7 m ³ / m ² filter area, preferably 0.773 m ³ / m ² filter area to 2.5 ₂ 6 m ³ / m ² filter area, each per minute . The filters according to the invention can be produced in a manner known per se, for example by a process as described in US Pat. No. 3,893,917. According to this known method, an open-cell, flexible organic foam material made of a multitude of materials is bonded to one another those cavities surrounded by a network of the foam material are impregnated with an aqueous slurry of a ceramic material, so that the foam material is covered with the slurry and the cavities are filled with it. The slurry made of ceramic material is selected so that after drying and heating the composition of the filter according to the invention results. About 80% by weight of the ceramic aqueous slurry is then removed from the impregnated foam material, for example by squeezing. When squeezing, the foam material is deformed and then allowed to return to its original shape. The remaining amount of slurry is evenly distributed through the foam material and covers the inner surface evenly. Individual pores can remain closed in an even distribution, creating more tortuous flow paths. Then the foam material covered with slurry is first dried, then heated to remove the flexible organic foam material, and finally the remaining ceramic coating is heat-treated or sintered. A solid ceramic foam structure is created from a large number of interconnected cavities, which are surrounded by a network of chemically bonded or sintered ceramics and represent an exact copy of the original organic material. The main component of the ceramic material from which the filters according to the invention are produced is A1203. In the present invention, the Al ₂ O _{3 is contained} in two forms. Mainly calcined Al203 is used in a grain size of 100 mesh to 500 mesh, preferably 325 mesh, in amounts of 55 to 70% by weight, preferably in amounts of 60 to 65% by weight. Al ₂ O ₃ is particularly suitable for use in ceramic filters, since it is not attacked by molten aluminum or copper, in contrast to, for example, silicon dioxide. In addition to the chemical resistance of the aluminum oxide, it also has the necessary mechanical and structural strength to meet the special requirements. A1 ₂ 0 ₃ is also contained in the form of micronized reactive aluminum oxide in amounts of 2 to 10% by weight, preferably 2 to 5% by weight. The use of the micronized reactive aluminum oxide in the amounts indicated is crucial in the present invention. The micronized reactive aluminum oxide provides an evenly distributed binder phase and a suitable rheology of the slurry. At the same time, additions of plastic materials to the slurry, which can show reactions with the molten metal, can be kept to a minimum. Montmorillonite, contained in amounts of 1 to 5% by weight, preferably 1 to 3% by weight, is another important component of the ceramic material. Montmorillonite is a highly plastic material with the approximate composition A1 ₂ 0 ₃ '4 Si0 ₂ ' H ₂ 0. It has been found, for example, that montmorillonite is considerably more plastic than bentcnite. The use of a relatively small percentage of highly plastic montmorillonite instead of bentonite, with a suitable rheological behavior of the slurry, results in a significantly improved strength, inherent in the ceramic composition, in the finished filter materials. Montmorillonite works in synergy with the micronized reactive aluminum oxide in order to achieve an improved strength, inherent in the ceramic composition, with a uniformly distributed binder phase.

Another essential component of the ceramic material is the presence of ceramic fibers in amounts of 1 to 10% by weight, preferably 1 to 3% by weight. More than 10% by weight of ceramic fibers cause them to clump together in the slurry and a uniform suspension cannot be achieved. More than 3% by weight of ceramic fibers only have a weak strengthening effect. The preferred 1 to 3% by weight of fibers bring about be with the same good dispersion of the fibers without lump formation in the slurry significant improvement in the strength of the finished filter elements. The ceramic fibers delay the formation of cracks and thus improve the strength inherent in the ceramic composition. The ceramic fibers used for cost reasons are made of aluminum silicate. Fibers, for example made of aluminum oxide or zirconium oxide, can also be used. Furthermore, the ceramic material contains 2.5 to 25% by weight of an air-binding agent which is essentially indifferent to molten metal. The air setting agent or binder binds or hardens the ceramic slurry without the use of heat, preferably by drying, usually by a chemical reaction when heated to moderate temperatures. The preferred agent is aluminum orthophosphate, suitably in 50% aqueous solution. Other air-binding agents that can be used are, for example, magnesium orthoborate, aluminum hydroxychloride, etc. Alkali metal silicates, such as sodium silicate, can be used at least in part, but these compounds are less desirable because they already melt at about 800 ° C and thus theirs Lose firmness. Furthermore, the silicon and any sodium contained can dissolve in the melt. For the same reasons, ethyl silicate and other phosphates could be used, but are also less desirable. Aluminum orthophosphate is particularly suitable because it contains a good combination of properties such as non-reactivity, heat resistance and setting ability. As described above, the air setting agent is added in the form of an aqueous suspension, in the case of aluminum ortho. phosphate preferably as a mixture of equal parts by weight of binder and water. The purpose of the binder is to give the ceramic molded body the necessary strength after the organic foam material has burned or volatilized and before the ceramic bond has been formed. The binder should also be used for the End product necessary strength can be achieved. The stability and strength of the chemical bonding of the preferred binders is sufficient for many applications of the end products, so that sintering at high temperatures is unnecessary. This strength is decisive and is maintained over a wide temperature range. 15 to 25% by weight aluminum orthophosphate is preferably used. As already stated, the ceramic material slurry is an aqueous slurry in such a concentration that the viscosity can be controlled and which enables the slurry to be processed. As a rule, 10 to 40% by weight of water is contained in the slurry, at least part of the water being introduced with the aqueous suspension of aluminum orthophosphate. The end products are bonded ceramic foam structures with a large number of interconnected cavities, which are surrounded by a network of ceramics. The production method allows the filter materials proposed according to the invention to be produced easily and simply in a wide variety of variations. For example, the ceramic filter according to the invention and its production method are suitable for producing filters with the properties as claimed in US Pat. No. 3,962,081. Further variations in terms of air permeability, porosity, pore size and thickness can also be achieved with the present method. It is particularly advantageous that the filter material according to the invention has a high temperature resistance and withstands the attacks of molten metal. Furthermore, no large metallostatic head is necessary when starting the filtration process. The following example illustrates the present invention:

example

wurde ein quadratisches Stück Polyurethanschaum mit einer Dicke von 5cm und 12 Poren je linearen cm eingetaucht. Das Schaumstück wurde geknetet, um die Luft daraus zu entfernen, die Poren mit Aufschlämmung zu füllen und die innere Oberfläche des Schaumes mit Aufschlämmung zu bedecken. Aus dem imprägnierten Schaum wurde ca. 80% der Aufschlämmung durch Abpressen in einem fest eingestellten Walzenstuhl entfernt. Das Schaumstück wurde dann getrocknet und erhitzt, wobei eine offenzellige Schaumstruktur aus keramischem Material resultierte, die genau dem Polyurethanschaum entsprach. Dieses Filterelement wurde dann mit einem Filterelement verglichen, das entsprechend der Lehre aus Beispiel 1 der US-PS 3 947 363 aus einer Aufschlämmung aus 47 Gew.-% A1203, 13 Gew.-% Cr₂O₃, 3.5 Gew.-% Kaolin, 1.0 Gew.-% Bentonit und 14.5 Gew.-% Aluminiumorthophosphat als 50 %ige wässrige Lösung hergestellt wurde. Die Druckfestigkeit des Filterelementes nach vorliegender Erfindung war um 50% grösser als des Al₂O₃/Cr₂O₃-Filterelementes, die Bindefestigkeit war um 10 bis 20% höher. Das Filterelement nach vorligender Erfindung liess sich für die Filtration sowohl von geschmolzenem Aluminium als auch Kupfer bei Resultaten, die mit dem A1203/Cr203-Filterelement vergleichbar waren, einsetzen.In an aqueous slurry of:

a square piece of polyurethane foam with a thickness of 5 cm and 12 pores per linear cm was immersed. The foam piece was kneaded to remove the air therefrom, to fill the pores with slurry, and to cover the inner surface of the foam with slurry. About 80% of the slurry was removed from the impregnated foam by pressing in a fixed roller mill. The foam piece was then dried and heated, resulting in an open cell foam structure made of ceramic material that exactly matched the polyurethane foam. This filter element was then compared with a filter element which, according to the teaching of Example 1 of US Pat. No. 3,947,363, consists of a slurry of 47% by weight A1203, 13% by weight Cr ₂ O ₃ , 3.5% by weight kaolin , 1.0% by weight bentonite and 14.5% by weight aluminum orthophosphate as a 50% aqueous solution. The compressive strength of the filter element according to the present invention was 50% greater than that of the Al ₂ O ₃ / Cr ₂ O ₃ filter element, and the binding strength was 10 to 20% higher. The filter element according to the present invention could be used for the filtration of both molten aluminum and copper with results comparable to the A1203 / Cr203 filter element.

Claims (10)

1. Porous ceramic filter for the filtration of molten metals, from a plurality of interconnected cavities, which are surrounded by a network of ceramic, containing 55 to 70 wt .-% Al ₂ O ₃ , 2 to 10 wt .-% micronized reactive Aluminum oxide, 1 to 5% by weight of montmorillonite and 1 to 10% by weight of ceramic fibers. a 2. A ceramic filter according to claim 1 containing 60 to 65 _G ew .-% Al ₂ O _3. 3. Ceramic filter according to claim 1 and 2, containing 2 to 5 wt .-% micronized reactive aluminum oxide 4. Ceramic filter according to claim 1 to 3, containing 1 to 3 wt .-% ceramic fibers. 5. Ceramic filter according to claim 1 to 4, containing ceramic fibers made of aluminum silicate. 6. Ceramic filter according to claim 1 to 5, containing 2 to 25 wt .-% of annealing products of an air-binding agent which is substantially indifferent to molten metal. 7. Ceramic filter according to claim 6, containing as glow products of an air-binding agent, the glow products of 15 to 25 wt .-% aluminum orthophosphate. 8. A method of manufacturing porous ceramic filters according to claim 1 by impregnating a flexible organic foam material with an aqueous slurry of ceramic material, drying the impregnated foam material, heating to remove the organic foam material, and then heat treating or sin tern of the resulting ceramic body, characterized in that the ceramic material 55 to 70 wt .-% Al ₂ O ₃ , 2 to 10 wt .-% micronized reactive aluminum oxide, 1 to 5 wt .-% montmorillonite and 1 to 10- Wt .-% contains ceramic fibers. 9. The method according to claim 8, characterized in that the aqueous slurry contains 60 to 90 wt .-% ceramic material. 10. The method according to claim 8 and 9, characterized in that the ceramic material 60 to 65 wt .-% Al ₂ O ₃ , 2 to 5 wt .-% micronized reactive aluminum oxide, 1 to 5 wt .-% montmorillonite, 1 to Contains 3 wt .-% aluminum silicate fibers and 15 to 25 wt .-% aluminum orthophosphate.